1. Technical Field
The present invention relates to a material testing machine, and more particularly, to an electrohydraulic servo-controlled material testing machine.
2. Related Art
An electrohydraulic servo-controlled material testing machine is known which is comprised of a controller for supplying, via a servo amplifier, an electric signal input to an electrohydraulic servo valve provided between a hydraulic power source and a hydraulic actuator such as a hydraulic cylinder, the electric signal input varying in accordance with a target displacement of a movable part of the actuator. Typically, the quantity of fluid output from the servo valve varies in response to the electric signal input, and the movable part of the hydraulic cylinder is displaced at a speed proportional to the fluid quantity, whereby a load is applied to a test piece held between the cylinder movable part and a main body of the testing machine. An actual displacement of the cylinder movable part is detected and is supplied as a feedback signal to the controller. Under the control of the controller, a feedback control is carried out to cause the actual displacement to close to a target displacement.
In this specification, the term "displacement of a test piece" indicates the displacement of one end of the test piece coupled to a movable part of an actuator which is caused by the displacement of the movable part of the actuator. In a material testing machine of a type provided with two actuators whose movable parts hold a test piece therebetween and are typically displaced in opposite directions, the term "displacement of the test piece" indicates the sum of displacements of opposite ends of the test piece caused by the displacement of the two movable arts of the actuators. That is, the term "displacement of the test piece" indicates deformation of the test piece caused by the isplacement of the movable part(s) of the actuator.
The term "servo system" indicates a system mainly comprised of an actuator, a servo amplifier, and a servo valve. The term "control system" indicates a system mainly comprised of a servo system and a controller for controlling the operation of the servo system. The term "control loop" or "feedback control loop" indicates a loop mainly comprised of a servo system, a controller, and a test piece. Moreover, the term "force control system" indicates a control system for carrying out a feedback control with use of an actual force applied to the test piece, as the controlled variable, whereas the term "displacement control system" indicates a system for executing a feedback control using, as a controlled variable, an actual displacement of the test piece. The term "load" indicates a broadly defined load which includes a force applied to the test piece and generally referred to as a load, and which also includes the displacement of the test piece. In case that the actual load or the actual displacement is referred to as a controlled variable, the term "control objective value" or "control target value" indicates a target load.
A typical electrohydraulic servo-controlled material testing machine performs material testing, while controlling the actual load given to the test piece (the actual force applied to or the actual displacement of the test piece) to the target load in a feedback manner. Thus, it is preferable to optimize the control gain for the feedback control, thereby performing the material testing efficiently and stably because the control response lowers so that much time is required for material testing if the control gain is excessively small, whereas hunting phenomenon occurs so that the stability and reliability of the testing are impaired, sometimes leading to test piece breakage if the control gain is excessively large.
The optimum control gain for the feedback control varies depending on mechanical properties, especially stiffness (elastic constant), of the test piece. For this reason, in the case of material testing as for a test piece whose stiffness is unknown, preliminary testing is heretofore carried out repeatedly while changing the control gain, to thereby determine the optimum control gain in a trial and error manner. This requires labor and time.
Moreover, the stiffness of the test piece varies in dependence on the load given to the test piece. Especially, the stiffness largely varies if the test piece is in its elastic deformation region. In other words, the optimum control gain for feedback control may change as the stiffness changes, even in the course of material testing, causing the feedback control to be improper so that the testing efficiency and stability may be impaired.
An electrohydraulic servo-controlled material testing machine may be of a type having a force control system for feedback-controlling the actual force applied to the test piece to the target force and a displacement control system for feedback-controlling the actual displacement of the test piece to the target displacement, and adapted to select a desired one of these control systems to carry out the feedback control in a force control mode or a displacement control mode. Generally, either one of the control modes is selected during the testing in accordance with instructions given by an operator.
In a testing machine of a type having a controller for controlling the operation of the servo system, the control gain for the control loop which is comprised of the servo system, the controller, and the test piece, especially the control gain for the controller largely differs according to whether the testing machine operates in the force control mode or in the displacement control mode. That is, the control gain for the controller is set to be small in the force control mode, whereas it is set to be large in the displacement control mode. Moreover, the mechanical properties, especially the stiffness of the test piece generally varies in dependence on the load given to the test piece, and the proper control gain greatly changes depending on the mechanical properties of the test piece. If the force control mode is changed over to the displacement control mode in a condition where the load is kept applied, the applied load abruptly changes attributable to the difference of control gain between the two control modes, so that a large shock may be applied to the test piece.
Upon changeover of the control mode, heretofore, the operation of the servo system is temporarily stopped to release the load given to the test piece, and the servo system is operated in the control mode after the changeover. The material testing is interrupted each time the control mode is changed over, so that the testing efficiency lowers, and efforts to release the load are troublesome.